In an OFDM based cellular communication system, a mobile station (MS) may receive downlink-transmitted signals from different cells and may need to differentiate these signals by using different cell codes. For example, in an OFDM-CDMA cellular system, the downlink-transmitted signals from different cells are differentiated by using scrambling codes (cell codes), thereby allowing for reuse of frequency and spreading codes in contiguous cells. As such, an MS terminal, when switched on, needs to search for a cell (i.e., synchronizing to the associated downlink scrambling code) before any communication. This procedure is known as initial cell search. On the other hand, during active or idle modes of an MS terminal, searching for a cell is also needed for identifying handoff candidates. This procedure is known as target cell search. The performance of cell search method directly impacts the perceived switch-on delay, link quality and power consumption of an MS. Therefore, cell search is important for the design of OFDM based cellular communication systems.
For brevity, cell search methods here are described in terms of only multi-carrier CDMA (MC-CDMA) system, one type of OFDM-CDMA systems, although they may also be applied to other OFDM based systems. Conventional cell search methods for MC-CDMA cellular systems include two types of methods, synchronization channel (SCH)-based method and common pilot channel (CPICH)-based method, in which cell search procedure is highly dependent on the frame structure of downlink-transmitted signal.
Consider that there are J downlink scrambling codes, denoted by C(i)[k], k=0˜K−1, i=1˜J, allowing for unique cell identification in every cluster of J cells where K is the length of the scrambling codes. Assume that cell j with the scrambling code C(j)[k] is the desired cell to be searched for. Typically, the J cells are further divided into several groups to reduce the number of scrambling codes to be searched for, where each group is represented by a group code.
FIG. 1 shows the frame structure of the SCH-based cell search method. Each frame consists of M OFDM symbols. Each OFDM symbol consists of not only NFFT-sample useful data but also NGI-sample cyclic prefix (CP), namely, guard interval (GI), for avoiding intersymbol interference (ISI) as well as inter-carrier interference (ICI). Accordingly, the length of an OFDM symbol is NOFDM=NFFT+NGI. The downlink-transmitted signal in FIG. 1 includes three types of signals, CPICH signal, SCH signal, and traffic channel (TCH) signal. CPICH signal contains the information about the scrambling code, while SCH signal about the group code and frame timing. TCH signal is used for transmitting TCH data. In the transmitter of the base station (BS) in cell j, the data of TCHs and CPICH are spread in frequency domain by different spreading codes, and then added and scrambled by the scrambling code C(j)[k]. The scrambled signal is further combined with SCH signal, modulated via an NFFT-point inverse discrete Fourier transform (IDFT) (or, more efficiently, inverse fast Fourier transform (IFFT)), and inserted with GI to generate the downlink-transmitted signal. The number of sub-carriers is exactly identical to the length of the scrambling code (K), and the IFFT size NFFT≧K.
In the receiver of an MS, the received signal is processed by the cell search procedure shown in FIG. 2. The procedure involves three steps: (S1) symbol synchronization to detect OFDM symbol timing (OFDM symbol boundary), (S2) frame synchronization and group identification to detect flame timing (flame boundary) and the group code, and (S3) scrambling-code identification to detect the scrambling code C(j)[k]. In step S1, the symbol timing is detected by using the correlation property of CP. In step S2, after removing GI from the received signal and performing NFFT-point discrete Fourier transform (DFT) (or, more efficiently, fast Fourier transform (FFT)), the frame timing and group code are simultaneously detected by using SCH signal in frequency domain. In step S3, the scrambling code C(j)[k] is identified from the detected group by using CPICH signal, and verification is conducted to avoid false detection, thereby minimizing unnecessary MS activities.
Because SCH signal is not orthogonal to TCH signal and CPICH signal, cell detection performance of the SCH-based method is degraded due to the interference from TCH signal and CPICH signal, and data detection performance is also degraded due to the interference from SCH signal. For this reason, the CPICH-based method (to be described next) does not include SCH signal into the frame structure, and thus performs much better than the SCH-based method.
FIG. 3 shows the frame structure of the CPICH-based cell search method. Each frame consists of M OFDM symbols. Each OFDM symbol of length NOFDM samples consists of NFFT-sample useful data and NGI-sample CP (GI). The first and last OFDM symbols, indicated by CPICH1 and CPICH2, respectively, correspond to CPICH signal, while the remaining (M−2) OFDM symbols are used for transmitting TCH data, where RCPICH is the power ratio of CPICH signal to the signal of one TCH. CPICH signal contains the information about the scrambling code, group code and frame timing. Because CPICH signal and TCH signal are allocated in different OFDM symbols (i.e., different time slots), no interference between them is incurred. Similar to the SCH-based method, the receiver of an MS for the CPICH-based method also uses the three-step cell search procedure shown in FIG. 2. The only difference is that in step S2, the frame timing and group code are simultaneously detected by using CPICH signal, instead of SCH signal, in frequency domain.
Recall that step S1 for both the SCH-based and CPICH-based methods is performed in time domain, while steps S2 and S3 in frequency domain using NFFT-point DFT (or FFT). In step S2, a lot of candidates for detecting the frame boundary need to be tested in frequency domain to find an optimum one. This implies that step S2 requires a lot of DFT (or FFT) operations for frame synchronization. Accordingly, the conventional cell search methods require high computation complexity. Furthermore, cell detection performance of the CPICH-based method is sensitive to channel effects because of a restrictive assumption for channel response in step S3. When this assumption is not satisfied that is the typical case in practical application, it may lead to false detection.
In the European patent application EP0940942, a synchronization preamble and synchronization protocol for an MC-CDMA mobile communication system is disclosed. The communication method enables remote stations to synchronize in time and frequency to their serving base station. It enables a base station and its remote stations in a cell to synchronize in a noisy environment where signal is interfered by other base stations and remote stations in other cells. One major drawback of the communication method is the cell detection performance also sensitive to channel effects.